Unknown Lab Report
April 25th, 2006

Introduction The purpose of this lab was to identify two unknown bacteria cultures using various differential tests. The identification of these unknown cultures was accomplished by separating and differentiating possible bacteria based on specific biochemical characteristics. Whether the tests performed identified specific enzymatic reactions or metabolic pathways, each was used in a way to help recognize those specifics and identify the unknown cultures. The differential tests used to identify the unknown cultures were oxidase, catalase, lactose and sucrose fermentation, Kugler/iron agar, nitrate reduction, gelatin hydrolysis, starch hydrolysis, manitol salt, MR-VP, citrate, bile esculin, …show more content…

Iodine was the dropped on the cultures to react with the starch and form a dark brown color. The observance of a yellow halo around the culture indicates that the starch was hydrolyzed and the bacteria were positive for starch hydrolysis. No halo constitutes a negative test. Manitol salt agar is both a selective and differential media used for gram (+) cocci. It is selective for salt tolerance and differential for maintol sugar fermentation. It also contains phenol red, which acts as a pH indicator, turning yellow under acidic conditions. The agar is most often used for the selection of S. auerus. Growth and a yellow color change are positive test results. No growth is a negative test result. Methyl red (MR) is a test used to detect organisms capable of overcoming an added phosphate buffer in the medium to lower the pH of the broth. The test was performed only on gram (-) bacteria, as it mostly tests for enterics who can do this by performing mixed acid fermentation. After inoculation, the broth is incubated for 5 days and then methyl red is added. A red broth is a positive test result, whereas a yellow or orange broth is a negative test result. Voges-Proskauer (VP) is a test used to detect organisms that ferment but quickly convert their acid products to acetoin. Addition of the VP reagents (KOH and á-napththylamine) oxidizes acetoin to diacetyl, which in turn reacts with guanidine nuclei to produce a red color. A positive VP